The invention relates to an electro-optical display element with a very small dependence of the contrast on the angle of observation and with a particularly steep electro-optical characteristic curve.
The properties of nematic or nematic-cholesteric liquid-crystalline materials are utilized for liquid-crystal display elements in order to effect significant changes in their optical properties, such as light transmission, light scattering, birefringence, reflectance or color, under the influence of electric fields. The functioning of display elements of this type is based here, for example, on the phenomena of dynamic scattering, the deformation of aligned phases or the Schadt-Helfrich effect in the twisted cell.
Among these liquid-crystal display elements, twisted nematic cells (TN cells) have gained particular importance recently, because they can be operated with relatively low control voltages which can be readily made available even by small batteries. Moreover, these display elements have hitherto been the best which can be constructed as matrix display elements which allow the presentation of a high information density without an intolerably large numer of control lines, lead-ins and lead-outs.
When used in practice, however, the TN cell, in particular in the form of matrix display elements, still raises difficulties. These include above all the more or less pronounced dependence of the contrast on the angle of observation and the restricted multiplexing capacity.
Proposed solutions for both problems are already to be found in the literature, and some of these have been put into practice. Thus, for example, the dependence of the contrast on the angle of observation in a TN cell can be significantly improved by suitable choice of the refractive index anisotropy .DELTA.n and of the cell thickness d [L. Pohl, G. Weber, R. Eidenschink, G. Baur and W. Fehrenbach, Appl. Phys. Lett. 38 (1981) 497].
Particularly in the so-called first transmission minimum according to Gooch and Tarry [C. H. Gooch and H. A. Tarry, J. Phys. D8 (1975) 1575], the dependence of the contrast on the angle of observation is at a minimum. This requires the condition ##EQU1## to be met (.lambda.=wavelength of the light used).
The multiplexing capacity of a TN cell is determined by the liquid crystal parameters K.sub.1,K.sub.2,K.sub.3 (elastic constants for spreading, twisting and bending), by the dielectric constants .epsilon..sub..perp. and .epsilon..sub..parallel., and by the refractive indices n.sub.o and n.sub.e, and also by cell parameters, such as, for example, the twist angle and tilt angle at the surface of the substrate. The influence of these material parameters and cell parameters has already been investigated by several authors [D. W. Berreman J. Appl. Phys. 46 (1975) 3746; F. Gharadjedaghi and J. Robert, Rev. Phys. Appl. 11 (1976) 467; G. Baur, in "The Physics and Chemistry of Liquid Crystal Devices" (edited by G. J. Sprokel) Plenum, N.Y., (1981) pages 61 et seq.; G. Baur, Mol. Crystl. Liq. Cryst. 63 (1981) 45; C. Z. van Doorn, C. J. Gerritsma and J. J. M. J. de Klerk, in "The Physics and Chemistry of Liquid Crystal Devices" (edited by G. J. Sprokel) Plenum, N.Y., (1980), pages 95 et seq.; F. J. Kahn and H. Birecki, in "The Physics and Chemistry of Liquid Crystal Devices" (edited by G. J. Sprokel) Plenum, N.Y., (1980a), (1980b), pages 125 et seq].
According to an approximation equation, the multiplexing capacity is estimated by ##EQU2## [M. Schadt and P. R. Gerber, Zeitschrift Naturforsch. 37a (1982) 165].
The smaller p, the higher the multiplexing capacity. According to this estimation, only the K.sub.3 /K.sub.1 ratio and .DELTA.n.d affect the steepness of the electro-optical characteristic curve and hence the multiplexing capacity. The K.sub.2 /K.sub.1 and (.epsilon..sub..parallel. -.epsilon..sub..perp.)/.epsilon..sub..perp. =.DELTA..epsilon./.epsilon..sub..perp. ratios are disregarded.
Experimental investigations of TN cells with the liquid crystal materials hitherto predominantly used have shown that the steepness of the electro-optical characteristic in the second transmission minimum is better than that in the first minimum, whereas the dependence on the angle of observation in the first minimum is better than that in the second transmission minimum.
By means of the proposed solutions hitherto known, the most important difficulties arising in the construction and use of TN cells can admittedly be solved individually, but not simultaneously.
The disclosures of all of the references cited above are incorporated by reference herein.